Silicon solar cells are the most common type of photovoltaic conversion device today, comprising over 90% of the solar cell market. These devices are typically made using full wafers of silicon, and the silicon material is therefore a significant component of their cost. Given the cost, there is a motivation for decreasing the thicknesses of silicon wafers. For reference, a silicon wafer used in integrated circuit processing is 700-800 μm thick. Currently, solar cell wafers are on the order of 200 μm thick, and the trend is toward reducing this thickness in half.
A typical solar cell factory processes high quantities of such wafers. A square wafer 205 mm on a side produces approximately 6.7 watts of electricity, assuming 0.1 w/cm2 incident power and 16% efficiency. Approximately 150,000 wafers must be processed to make 1 megawatt of solar cells. A typical factory produces 50 megawatts, equal to about 850 wafers per hour.
While the trend continues to make wafers thinner, one problem that makes it difficult to achieve and maintain such high processing throughput is the potential for wafer breakage. If a wafer breaks during processing, in many cases an operator must intervene to clean out the debris. Such events, if frequent, can seriously affect the line throughput and increase the cost of processing.
Some previous attempts at detecting defects in semiconductor wafers have been made. For example, U.S. Patent Pub. No. 2004/0206891 to Ma et al. describes a non-destructive process for detecting defects in a semiconductor wafer such as micropipes and screw dislocations by illuminating the wafer with polarized light. However, Ma et al. do not detail the possible light sources that are used, and the ability to transmit visible light through wafers comprised of materials other than the SiC material described by Ma et al. may be limited. So a reliable way of detecting defects in wafers comprised of other types of materials is not possible based on Ma et al.'s teachings. Moreover, Ma et al. are limited to analyzing sub-regions of a wafer and do not allow for rapid scanning of an entire wafer at a time. Still further, polarized light from Ma et al.'s system may enter a wafer at oblique angles, especially in compact systems, which can further degrade the performance of the polarization measurement.
Therefore, there remains a need for methods to detect defects in starting wafers that can lead to breakages, and particularly methods that can be implemented before or together with other in-line processes.